How to project a face to another face?
You can extrude from  a cut in the ceiling rather than trying to project the wall to it.


ShiftD duplicate off the top surface of your wall, and make a clean profile of it using X limited dissolve, but not so harshly as to damage the curvature.





From orthographic Z, Header Menu > Mesh > Knife Project the profile
onto the ceiling, checking 'Cut Through'





With Snap set to Vertex and Active, and a convenient vertex active, EZ Extrude a duplicate of the resulting selection in the ceiling downwards in 2 stages, snapping on the way.. (Take care not to create internal faces)





SZ0 scale the bottom of the wall to 0 about its median, AltM > By Distance merge out redundant vertices.. You can tidy up again with Limited Dissolve.)